MMO's aren't dependent upon graphics, really. Their hook is the grind, the game's mechanics, and the social aspects. Hell, the direct ancestors of MMO's, MUDs, had no graphics at all and people still play them today.
FPS on the other hand are completely the reverse, an FPS with bad graphics better have something else pretty awesome going for it. Not just FPS either. third person shooters, third person action, same deal. For some genres (and especially for single-player games) it's an accepted part of making a successful title.
Also, I'd like to draw a distinction here between "realistic graphics" and "good graphics". They aren't the same. Stylization is a conscious decision of art direction in a game, lots of games *could* have much more "realistic" graphics but go with another look intentionally. The Walking Dead, for example, has comic book-esque graphics and that was something they did on purpose, not because they just couldn't make it look more realistic. That cel-shaded Zelda game is another example. Actually, any game with cel-shading. Borderlands. 13. Games that emulate the 8-bit look of old RPG/adventure titles...
Objection, Your Honor. He's reciting the opening to Quantum Leap.